I want a Moonshadow expansion... and expansions are definitely my preferred form of DLC.
Ditto this.
My idea for an expansion (other than the above) :
24 hours after installing (like SI in Oblivion), you get a new conversation topic: the "Greatbeast". Choose this option in conversation with certain people and they'll explain how people are vanishing in a certain settlement and their bodies found horrifically mauled, along with livestock and other animals. It seems to be the work of the mythical Greatbeast. You travel to the settlement and try and hunt down the creature. Eventually you find and slay it... beginning the new storyline, and teleporting you to Hircine's realm (I think it's Hircine).
Basically Hircine has decided to arrange a grand hunting tournament for a bit of fun. By sending his Greatbeasts all over Tamriel, he has gathered each nation's greatest hunters and summoned them to his realm. He wishes to determine the greatest mortal huntsman.
By day you compete in challenges -- hunting a particular beast and so on -- and you can attempt the challenge as many times as you like until you die trying. By night the roles are reversed, and the hunters become the prey. You must form alliances to survive the predators that swarm the land by night -- but be warned! Other competitors may try to thin the competition out by killing off the other hunters! Do you have what it takes to win the tournament and be crowned the greatest hunter?
^^^ I think the idea of forming and breaking alliances would really add something. You might befriend a mage-type character and convince them to team up so they can heal you, only to betray them and stab them in the back when it suits you -- unless they get you first. As an ultimate reward for the questline, I think it'd be nice to have some sort of unique mount to take back to Skyrim -- perhaps a sabre-toothed tiger? Unlikely to happen, but I'd love it.